This proposal has two main objectives: a) to determine the manner in which muscles and populations of fibers within muscles are recruited during locomotion: 1) as animals increase speed and change gaits; and 2) as animals use their muscles for different tasks, i.e., positive mechanical work and elastic storage of energy. b) to investigate the contribution of skeletal muscle to total resting metabolism. We have been using the following techniques: EMG's synchronized with high speed movies of running animals to determine the activity and length changes of active muscles; glycogen loss from muscle fibers to determine the cross-sectional area of active muscles, populations of active fibers, and the power of active muscles; length changes of tendons as a means of measuring force exerted by a muscle during locomotion; blood flow to muscle groups together with oxygen extraction from the blood to measure their oxygen consumption; 24 hour metabolic rates of mammals whose resting metabolism and maximum aerobic metabolism vary by approximately 5-fold; and the energetics and mechanics of isolated muscles (using temperature as a variable to alter intrinsic velocity of the muscle). In the coming year we will attempt to develop a labeled deoxyglucose technique to study muscle fiber recruitment in vivo. We hope this technique will provide greater sensitivity and finer temporal resolution of fiber recruitment patterns than the glycogen loss technique tht we have been using.